The Building Blocks

PowerPoint PresentationDon’t cross membranes.
Contain DNA within nucleus
AP Biology
RNA & DNA
across chains
spiraled in a double helix
double helix 1st proposed as structure of DNA in 1953 by James Watson & Francis Crick
(just celebrated 60th anniversary in 2013!)
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polymer grows in one direction
N bases hang off the
sugar-phosphate backbone
Dangling bases?
The 2 strands are complementary.
One becomes the template of the other & each can be a template to recreate the whole molecule.
AP Biology
Information polymer
like the letters of a book
stored information is passed
from parent to offspring
need to copy accurately
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All other biomolecules we spoke about served physical or chemical functions. DNA & RNA are information storage molecules.
DNA well-suited for an information storage molecule:
chemically stable
stores information in the varying sequence of nucleotides (the genetic code)
its coded sequence can be copied exactly by the synthesis of complementary strands; easily unzipped & re-zipped without damage (weak H bonds)
damage to one strand can be repaired by addition of bases that match the complementary strand
AP Biology
AP Biology
DNA molecule
Double helix
A :: T
C :: G
H bonds?
H bonds = biology’s weak bond
• easy to unzip double helix for replication and then re-zip for storage
• easy to unzip to “read” gene and then re-zip for storage
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Copying DNA
have one, can build other
have one, can rebuild the whole
Matching halves?
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when cells divide, they must duplicate DNA exactly for the new “daughter” cells
Why is this a good system?
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When does a cell copy DNA?
When in the life of a cell does DNA have to be copied?
cell reproduction
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when cells divide, they must duplicate DNA exactly for the new “daughter” cells
Why is this a good system?
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DNA replication
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
James Watson
Francis Crick
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1953 | 1962
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A chemist by training, Franklin had made original and essential contributions to the understanding of the structure of graphite and other carbon compounds even before her appointment to King's College. Unfortunately, her reputation did not precede her. James Watson's unflattering portrayal of Franklin in his account of the discovery of DNA's structure, entitled "The Double Helix," depicts Franklin as an underling of Maurice Wilkins, when in fact Wilkins and Franklin were peers in the Randall laboratory. And it was Franklin alone whom Randall had given the task of elucidating DNA's structure. The technique with which Rosalind Franklin set out to do this is called X-ray crystallography. With this technique, the locations of atoms in any crystal can be precisely mapped by looking at the image of the crystal under an X-ray beam. By the early 1950s, scientists were just learning how to use this technique to study biological molecules. Rosalind Franklin applied her chemist's expertise to the unwieldy DNA molecule. After complicated analysis, she discovered (and was the first to state) that the sugar-phosphate backbone of DNA lies on the outside of the molecule. She also elucidated the basic helical structure of the molecule.
After Randall presented Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick. The scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953. Franklin was not bitter, but pleased, and set out to publish a corroborating report of the Watson-Crick model. Her career was eventually cut short by illness. It is a tremendous shame that Franklin did not receive due credit for her essential role in this discovery, either during her lifetime or after her untimely death at age 37 due to cancer.
AP Biology
Interesting note…
biotech procedures
more G-C =
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AP Biology
Function
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peptide bond